This invention is directed to an improved composite chill cast iron roll of the type disclosed in U.S. Pat. No. 3,623,850, issued Nov. 30, 1971 to Paul J. Horvath, Jr. More particularly this invention relates to a new use of such rolls in the hot reduction of ferrous and ferrous alloy products, such as plates, strip, bars and rods. These rolls, as a result of the combination of chemistry, manufacturing sequence, and post treatment, may be characterized as composite martensitic, nodular graphite, chill-cast iron rolls.
Since the original development by Horvath, such rolls have gained much notoriety for their performance in cold mills, or in cold rolling applications. Their outstanding performance has been attributed to the ability of such rolls to resist marking, bruising and spalling, while being readily redressed for further service. However, with cold rolling applications, heat, hence thermal fatigue and cracking, is not a problem.
Rolling mill design has developed over the years into a complex science having many facets to it. For example, cold rolling applications, such as reviewed in U.S. Pat. No. 3,623,850, and hot rolling applications are just two of such facets. Different criteria must be used to determine roll design, such as material selection, properties and capabilities of the roll. In other words, a roll was designed for a specific rolling application because it possessed the properties needed for such application. As indicated above, resistance to thermal fatigue or cracking is a major consideration in determining suitability of a roll for use in hot rolling applications. As a consequence, roll manufacturers, when designing rolls for hot mill applications, particularly for the first several stands of a hot strip mill where the strip temperatures exceed about 1800.degree. F, maintained the shell hardness below a specified value.
In the publication, Roll Specifications For Finishing Stands of a Modern Continuous Hot Strip Mill, by John M. Dugan, published by the Association of Iron and Steel Engineers, copyright 1970, the author indicates that the shell hardness of the rolls in the initial stand vs. final stand of a hot mill finishing train will vary by about 7 points on the shore "C" hardness scale. That is, where the temperature of the strip is hottest, the lower hardness roll, i.e. about 75 shore C, is used.
While the shell hardness of a work roll is a prime consideration in the selection of a roll, there are others. For instance, the article entitled, "Cause and Prevention of Hot Strip Work Roll Banding," by Charles E. Peterson and published in the Iron and Steel Engineer Year Book, 1956, offers three possible solutions to the banding problem in a hot mill. Banding, as defined by the author "occurs primarily on the rolls of the first two finishing stands, [and] is caused by the adhesion of sizeable patches of scale on the roll surface. Generally the scale patches are elongated in the direction of rolling, giving the appearance of bands." His solution is (1) effective scale removal from the strip, (2) selection of roll material combining high hardness with freedom from graphite, and (3) adequate coolant to keep rolls as cold as possible.
Faced with these prejudices, the prior art settled for cast steel rolls, a graphite free roll having a nominal composition of 1.7C-- 1.0-Cr--1.7Ni-Fe. However, such rolls are limited in the quantity of product that can be rolled, and by the frequent dressing required to prepare the roll once again for service. To improve the usable work life of their rolls, roll makers began to look to high chromium rolls. Typically these rolls contain about 12 to 20% chromium and are characterized by a Shore-C hardness of between about 60 and 75. While the usable life of a hot mill roll had been increased with the introduction of the high-chromium roll, the premium cost of the highly alloyed chromium rolls made their selection a costly alternative. Moreover, the high chromium roll is sensitive to thermal conditions in the mill and thereby frequently requiring cutdowns because of excessive fire cracking.
Confronted by these facts, including the high cost of an alternative answer, a different approach was needed. It was discovered that a cast iron roll, having a shell portion containing nodular graphite in a martensitic matrix, with an average surface hardness of at least 76 Shore-C, could be used effectively in the hot reduction of ferrous and ferrous alloy products, such as strip, plates, bars and sheet. This was particularly dramatic where the temperatures of the workpiece exceeded about 1800.degree. F. Finally, the cost of such rolls was comparable to that of the presently used cast steel or cast iron rolls, and considerably below the cost of the chromium rolls.